Extensive carbonate crusts have formed on the tailings of the Woodsreef Asbestos Mine, sequestering significant amounts of CO 2 directly from the atmosphere. The physico-chemical (pH, T, conductivity), chemical (cations, dissolved inorganic carbon (DIC)) and isotopic (δ 2 H, δ 18 O, δ 13 C DIC , F 14 C) signatures of waters interacting with the tailings and associated carbonate precipitates provide insight into the processes controlling carbonation. We observe two distinct evolutionary pathways for a set of stream and meteoric-derived water samples, respectively, with both groups generally being characterised as moderately alkaline, bicarbonate-dominated and Mg-rich waters. Stream water samples are supersaturated with CO 2 and therefore prone to degassing, which, in combination with evaporation, drives carbonate supersaturation and precipitation. Isotopic signatures indicate soil CO 2 as the main carbon source in the stream waters entering the tailings pile, whereas water emerging downstream of the tailings pile may also contain carbon from the dissolution of isotopically light bedrock magnesite in an open system with respect to soil CO 2. The evolution of meteoricderived waters on the other hand, partly occurs under CO 2-limited conditions, which results from reduced CO 2 ingress at depth and/or a temporal lag between fluid alkalisation and kinetically hindered uptake of CO 2 into alkaline solution. A high pH, Mg-rich meteoric water absorbs atmospheric CO 2 after discharging into a tunnel within the tailings pile, resulting in high DIC concentrations with atmospheric carbon isotope signature. Evaporation of the water at the discharge point in the tunnel drives precipitation of hydromagnesite (Mg 5 (CO 3) 4 (OH) 2 •4H 2 O), displaying a clear atmospheric isotope signature, broadly consistent with previous estimates of carbon and oxygen isotope fractionation during precipitation of hydrated Mg-carbonate.
Mineral carbonation has the potential to store billions of tonnes of CO 2 safely and permanently. Enhancement of the kinetics of the formation of magnesium carbonate from magnesium-bearing silicate minerals has been the subject of numerous research studies. However, significant progress is yet to be achieved. This is, in part, due to a lack of understanding of the mechanism of the formation of magnesite in the presence of additives and under mineral carbonation conditions. In this work, an indepth study was performed to investigate the precipitation of magnesium carbonate during single step high pressure, high temperature carbonation of thermally activated serpentine in an aqueous bicarbonate solution. Slurry samples were obtained throughout the duration of the carbonation experiments, enabling analysis of both the aqueous and solid compositions over time, providing insight into the reaction mechanism. Additionally, the effect of operating temperature on the formation of various magnesium carbonate species was examined. TGA-MS, in combination with XRD and SEM, confirmed the formation of hydromagnesite in the absence of carbon dioxide (CO 2) during the reactor heat up period, owing to a reaction with the sodium bicarbonate (NaHCO 3) carrier solution. Hydromagnesite was transformed to magnesite over time, with the rate of this phase transformation highly dependent on the reaction temperature. At 185 °C all hydromagnesite converted to magnesite in a few minutes whereas at 120 °C even after 90 minutes hydromagnesite remained in the reactor. 2 PHREEQC thermodynamic software was used to assess the observed formation of carbonate species. The model prediction was consistent with the experimental results obtained in this work.
In this experimental work we synthesized, characterized, and investigated the reactivity of silica‐enriched residue materials produced from low pressure, low temperature dissolution applicable to a two stage mineral carbonation process. XRF and XRD analysis indicated that 66 wt % of silica‐enriched residue (SER) produced from dissolution of heat activated lizardite comprised amorphous silica. We also treated SER with nitric acid to produce amorphous acid treated silica‐enriched residue (ATSER) with over 88 wt % silica. Dissolving these synthesized materials in highly alkaline solution with pH similar to cement paste pore solution for 28 days showed that all synthesized materials displayed some level of Si solubility. ATSER showed the most rapid rate of Si extraction, higher than silica fume (a commercial pozzolanic cement substitute), followed by silica‐enriched residue (SER) which had a faster rate of Si extraction compared with Portland cement as a standard. © 2018 American Institute of Chemical Engineers Environ Prog, 38:e13066, 2019
This research explores the use of serpentinized dunite (which is comprised of 61% lizardite) as a feedstock for aqueous mineral carbonation. In initial experiments, dunite was heat‐activated (630°C, 4 h), adopting a procedure which is similar to that used for serpentinite to enhance their carbonation reactivity. Heat‐activation converts crystalline lizardite mineral into an amorphous, reactive phase, and the carbonation of this heat‐activated material resulted in a magnesite yield of 55% compared to 27% obtained with raw dunite under the same reaction conditions. The formation of silanol nests occurred during carbonation of heat‐activated dunite as deduced through FTIR and TGA‐MS analyses. Samples of dunite were also heat‐transformed at high temperatures (800°C, 3 h) to convert lizardite into forsterite, and these samples were also studied as potential feedstocks for mineral carbonation. Heat‐activated dunite was found to engender much higher magnesite yields compared to heat‐transformed dunite (forsterite rich) and raw dunite. This study suggests that during heat‐activation of dunite, as it is for lizardite, conditions should be maintained to avoid forsterite formation. © 2018 American Institute of Chemical Engineers Environ Prog, 38:e13075, 2019
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